Electronic systems typically require one or more regulated voltages to power various subsystems. A regulator is a circuit that may receive an input voltage and produce a regulated output voltage that may be at a different voltage level than the input voltage. One common type of regulator circuit is a low dropout regulator (“LDO”). An LDO regulator is a DC linear voltage regulator which can regulate the output voltage even when the input (or supply) voltage is very close to the output voltage.
One common problem associated with regulators, such as LDO regulators for example, is that the controller of a LDO may constitute a large portion of the entire LDO integrated circuit device area. This can be significant when the LDO is rated for small power handling. LDO controllers include circuitry to regulate and stabilize output voltage over a wide range of load currents including transient currents, as well as circuitry for overcurrent protection. A large compensation capacitor is also used in conventional designs for dominant pole and load tracking zero. Overcurrent protection is accomplished by a separate loop from the main loop. A separate soft start circuit is also needed to avoid inrushing current during LDO start up.
FIG. 1 depicts an example circuit diagram of a conventional LDO controller circuit configuration with separate current and voltage regulation loops according to the prior art. Circuit 10 includes an error amplifier 101, a buffer circuit 102, a compensator circuit 103, a feedback network 105, an overcurrent protection (“OCP”) control circuit 106, a pass transistor MPASS and a sensing transistor MSEN. The output VOUT of the LDO 10 is coupled with a capacitive load CLoad and a variable resistive load RLoad.
As shown, circuit 10 includes two separate loops for regulating the voltage and current in the LDO controller. The voltage regulation path VLoop includes a pass transistor MPASS coupled between the input voltage VIN and the output voltage VOUT. It also includes the error amplifier 101, buffer circuit 102, compensator circuit 103, and a feedback network 105 coupled with the pass transistor MPASS. The feedback network 105 provides a first input to the error amplifier 101, which is compared to a reference voltage Vref for driving the output Vamp_out of the error amplifier. The overcurrent protection path OCPLoop includes a current sensing transistor MSEN, the OCP control circuit 106, the error amplifier 101, the buffer circuit 102, and the compensator circuit 103. The output of the OCP control circuit 106 is provided to a control input of the error amplifier 101 to control the output signal Vamp_out of the error amplifier 101.
The disadvantages of the LDO controller circuit 10 include that it requires a substantial amount of integrated circuit area for all the circuit elements. The error amplifier 101, buffer circuit 102, and compensator circuit 103 each must be large in order to stabilize circuit 10 across process, voltage, and temperature variations, different output loading, and external capacitance and printed circuit board (“PCB”) parasitic variations within a single complicated loop. In addition, a separate OCP loop and a soft start circuit (not shown) are needed. These require extra load current sensing, OCP loop compensation, and Vref ramp circuitry. Further, due to the single loop nature of the LDO controller circuit 10, very high switching frequency is required if a duty cycle-based feedback network is used, which will increase dynamic switching losses.
Another example of a conventional LDO controller is shown in FIG. 2, which depicts an example circuit diagram of a Flipped Voltage Follower-based LDO controller circuit structure according to the prior art. In this configuration, there are two loops for stability and transient response. The main drawbacks of circuit 20 include that stability can only be achieved with very small capacitive load CL, and that the LDO cannot operate when large headroom is given—when VIN is higher than VOUT, VG is limited by VOUT+VGS (for transistor M9).